The present invention relates to an X-ray pattern transfer apparatus also referred to as the X-ray lithography apparatus for transferring a fine pattern formed in a mask to a wafer by using a soft X-ray.
As a hitherto known X-ray lithography apparatus, there can be mentioned the one disclosed in Japanese Patent Application Laid-Open No. 169242/1982 (JP-A-57-169242) which corresponds to U.S. Pat. No. 4,403,336. Referring to FIG. 3 of the accompanying drawings, this known X-ray lithography apparatus includes a gas chamber having an X-ray receiving window which is disposed on a side of an X-ray generating unit 1 where an X-ray exit window is provided. The gas chamber 2 accommodates therein a gaseous medium having a high transmissivity to the X-ray. A mask 3 having a desired pattern formed therein is mounted on the X-ray lithography apparatus 1 so as to cover an aperture formed in a wall of the gas chamber 2 at a position opposite to the X-ray receiving window. A wafer stage 5 is provided outside of the gas chamber 2 for holding a wafer 4 in an ambient atmosphere so as to be exposed to the X-ray through the mask 3, while allowing the wafer 4 to be moved relative to the mask 3.
Disposed within the gas chamber 2 is an optical system 6 which serves to aid positional alignment to be attained between the pattern of the mask and the wafer through visual observation. With a view to preventing the purity of helium gas filling the gas chamber 2 from being degraded due to admixing of the air possibly occurring upon replacement of the mask, a helium gas cylinder (bomb) 7 is provided in communication with the gas chamber 2 so that an amount of helium gas is constantly supplied to the chamber 2 with a corresponding amount of gas being discharged through a drain duct 8. However, the flow of helium gas within the chamber 2 is extremely small and can thus be neglected.
Further disposed within the gas containing chamber 2 are a light source for illumination of the viewing or detecting optics 6, a television camera 9, an optics positioning mechanism 10, a mask positioning device 11 and other. These components constitute heat generating sources within the gas chamber 2. Additionally, thermal flux produced by the soft X-ray generating unit 1 may flow into the gas chamber 2 through the X-ray exit window 12.
In the case of the hitherto known X-ray lithography apparatus reviewed above, no consideration is paid to removal of heat generated within the gas chamber 2 by the imager system and others mentioned above. Consequently, thermal distortion or deformation is likely to occur in the viewing or detecting optics, which results in occurrence of a drift of the detected values representative of the relative positions of the mask 3 and the wafer 4, giving rise to a problem that the accuracy of positional alignment is undesirably degraded. Under the circumstance, the X-ray lithography apparatus known heretofore can be used only after a warm-up operation which is continued until the temperature rise due to the heat generation has attained a saturated state. This warm-up operation usually takes several hours or more. Consequently, the X-ray lithography apparatus has to be operated continuously day and night in order to ensure a high working ratio. Further, such drift can occur when the operating condition is changed.
Parenthetically, it is noted that an X-ray lithography apparatus including means for preventing the temperature of a mask from being raised is disclosed in Japanese Patent Application Laid-Open No. 191433/1983 (JP-A-58-191433).